I've already searching how to modify save button and get some solution like this :

def create(self, cr, uid, vals, context=None):
        # Your logic goes here or call your method
        res_id = super(reservation, self).create(cr, uid, vals, context=context)
        # Your logic goes here or call your method
        return res_id

    def write(self, cr, uid, vals, context=None):
        # Your logic goes here or call your method
        super(reservation, self).write(cr, uid, id, vals, context=context)
        # Your logic goes here or call your method
        return True

I have Hotel module and class reservation. I want user will see this message warning

if self.duration < 0:
            return {
                'warning': {
                    'title': "Something bad happened",
                    'message': "It's very bad indeed",
                    }
                }

if they break some condition like let the guest name and room number being not filled or let check out date less than check in date. Where should I locate this warning code in that function?